The film by Britain's Channel 4 raises questions over the conduct of Sri Lankan forces during its final stages of operations against the separatist group, the Press Trust of India reported Tuesday.
It shows young Balachandran Prabhakaran, 12, in the custody of Sri Lankan troops sitting in a bunker, alive and unharmed. A few hours later, the boy is seen lying on the ground with bullet wounds in his chest.
The film, to be shown at next month's session of the United Nation's Human Rights Council in Geneva, has been called "lies, half-truths and numerous forms of speculation" by the Sri Lankan military.
The director of the documentary, Callum Macrae, said new evidence in the film is likely to increase pressure on the Indian government to support a resolution critical of Sri Lanka.